Docker Compose — Networking, Dependencies and Health Checks

Knowing how to declare services is only half the battle; making them talk reliably is the other half. In the previous lesson you saw that depends_on controls startup order but cannot tell whether a service is actually ready. This causes a notorious failure: the web container launches, fires a query at a database that is still booting, and crashes. Without a way to express readiness and to isolate which services may reach each other, multi-container apps become flaky and insecure. Compose answers this with custom networks for controlled connectivity, health checks that probe whether a service is truly alive, and condition-based dependencies that wait for health rather than mere existence. Together these turn a fragile stack into one that starts predictably every time.